Normally, the tank is of fair size and it is a slow process to heat all the water in the tank to a preset temperature. The water is not heated at the same rate that it is used, therefore, the rate of recovery for the water to reach again the desired temperature, is relatively slow. The storage tank provides a reserve of hot water which normally supplies short term needs. If more hot water is used than the amount of water stored in the tank, the temperature of the water drastically drops due to the heater's low heat recovery rate, then the user must stop the flow and wait for the heater to heat the water back to the desired temperature. This type of heater is usually installed in an environment where the ambient temperature is lower than that of the temperature of the water in the tank. Thus, the loss of heat to the ambient air causes the heater to turn on and continuously reheat the water in the tank in order to maintain the desired water temperature. The energy used to reheat the water is wasted and no benefit is derived from it.
Heretofore, numerous attempts have been made to reduce the heat loss and wasted energy. This includes obvious solutions such as insulation for the water heaters. This helped to reduce the heat loss to some extent but was not completely effective and adversely increased the size of the heaters known in the prior art. Another solution to this problem has been the introduction of a variety of tankless water heaters. These heaters reduced to some extent the problem of energy loss, but were characterized by insufficient volume of hot water and space problems. Obviously, even these tankless type water heaters brought on a new variety of problems. Most units available were of small capacity and had severely limited flow rates and temperature rise capability. The larger units attempted maximum flow rates and temperature rise but required excessively large minimum flow rates to energize the systems. Most depended on conventional flow detection devices to energize the heaters. Other shortcomings included were poor maintenance capability, inability to replace individually worn parts without substantial component replacement, and the inability to get rid of entrapped air or gases in the system. This was at times due to use of water wells as a source of water supply and to pressurized pump systems (i.e., to get rid of air or gases).